Undefined Constant

Why This Happens

Prior to PHP 8.0, using an undefined constant was treated as a string and raised a notice. In PHP 8.0+, it throws a fatal Error. This commonly happens when a config file that defines constants is not loaded, the constant name is misspelled, or you forgot to use quotes around a string.

The Problem

// config.php was not loaded
if (APP_ENV === 'production') {
    // ...
}

The Fix

require_once __DIR__ . '/config.php'; // Defines APP_ENV
if (APP_ENV === 'production') {
    // ...
}

// Or use a default:
$env = defined('APP_ENV') ? APP_ENV : 'development';

Step-by-Step Fix

1. 1

   Check the constant name

   Verify the constant name for typos. Constants are case-sensitive by default.

2. 2

   Ensure it is defined

   Check that define('APP_ENV', 'value') or const APP_ENV = 'value' is called before the constant is used.

3. 3

   Load the config file

   If the constant is defined in a separate file, ensure that file is loaded before the code that uses the constant.